Probing primordial 3He from hyperfine line afterglows around supercritical black holes

We consider the possibility of the detection of ³He II hyperfine line (rest frequency, 8.67 GHz) emission from ionized zones around accreting black holes (BHs) formed at high redshifts, z = 15-30. We show that the brightness temperature in 8.67GHz line increases and reaches a peak value after the accretion on to the BH exhausts and He III recombines into He II. This period of brightening last up to 40 Myr. We find that during this period the maximum brightness temperature reaches ≃ 0.2-0.5 μK, depending on the epoch when such a BH starts growing. The maximum angular size of the region emitting in the hyperfine line is around 0.5 arcmin. The flux from such a region (≃ 0.3 nJy) is too small to be detected by Square Kilometre Array1-MID (SKA1-MID). The rms of the collective flux from many emitting regions from a volume bounded by the synthesized beam and the bandwidth of SKA1-MID might reach 100 nJy, which is potentially detectable by SKA1-MID.